1. Field of the Invention
This invention relates generally to switchmode power supplies (SMPS) and, more specifically, to a start-up integrated circuit (IC) for a SMPS operating from high input voltages ranging from 40 V to 450 V.
2. Description of the Prior Art
An off-line switchmode power supply (SMPS) accepts a rectified AC input voltage and converts and regulates it to various desired output DC voltages. Common AC voltage sources are nominally 90 VAC, 120 VAC, and 240 VAC. The conversion and regulation is accomplished by a pulse width modulated (PWM) controller integrated circuit (IC). The majority of PWM controller ICs will operate with a maximum input voltage of 30 V only. External circuitry is therefore required for the SMPS to operate directly off the rectified AC line. The external circuit is called the start-up circuit. The start-up circuit is only used during start-up of the SMPS. Once the SMPS is started, an auxiliary voltage is generated and is used to power up the PWM controller IC. A typical power resistor discrete implementation of the start-up circuit uses a power resistor, a capacitor, and a zener diode.
PWM controller ICs typically have 2 current ratings: start-up current and operating current. Start-up currents are typically less than 1.0 mA whereas the operating currents are typically less than 17 mA. When the SMPS is powered up, the PWM controller IC will draw no more than 1.0 mA when its V.sub.cc voltage is below its start threshold voltage. A typical start threshold voltage is 16 V. Once the Vcc voltage becomes greater than the start threshold voltage, the PWM controller IC will typically draw 14 mA. The power resistor is selected such that it will provide the charging current for the external capacitor and the PWM controller IC's start-up circuit. The operating current is supplied by the capacitor across V.sub.cc. The PWM controller IC will continue to operate until its V.sub.cc voltage falls below the minimum operating voltage rating, typically 10 V. An auxiliary voltage is applied to the V.sub.cc of the PWM controller IC. It is desirable to have the auxiliary voltage to be greater than 10 V before the V.sub.cc capacitor discharges below the minimum operating voltage. When the auxiliary voltage is greater than 10 V, it will provide the required operating current for the PWM controller IC.
The problem with the power resistor approach is that power is continuously being dissipated by the power resistor after start-up. The power dissipation will be worst if the same design is used for both 120 VAC and 240 VAC operation. The high voltage start-up circuit of the present invention solves this problem by dissipating very little power after start-up. The start-up circuit monitors the output voltage and determines whether the SMPS has started up. After start-up, it turns off its internal high voltage input line to minimize power dissipation. Power is therefore only dissipated during start-up. This allows the start-up IC to be packaged in a cost effective three (3) terminal package.